1. Field of the Invention
The present invention relates to a biaxially oriented polyester film with improved scratch resistance.
2. Description of the Prior Art
Magnetic tape is produced from a biaxially oriented polyester film incorporated with a particulate organic or inorganic compound as a lubricant. There is an increasing demand for magnetic tape with good scratch resistance. Scratches are due to the rubbing of magnetic tape against metal or plastic guide pins, particularly in the case of high-speed dubbing. Scratches are also due to fine particles which have dropped off from magnetic tape during its run. Moreover, magnetic tape with poor scratch resistance is liable to drop-out because it easily wears in its manufacturing process. Therefore, improved scratch resistance is prerequisite for quality magnetic tape.
One of the known ways of improving scratch resistance is by the incorporation of polyester base film with fine particles as a lubricant. They may be inert ones having a specific hardness (e.g., Mohs hardness higher than 6) or particulate alumina taking a specific crystal form (e.g., .delta.-alumina or .gamma.-alumina). (See Japanese Patent Kokai Nos. 311131/89, 8238/91, and 6239/91.) The improvement in scratch resistance by this way is effective when magnetic tape is rubbed against plastic guide pins, but not so effective when it is rubbed against metal guide pins during high-speed dubbing. This indicates that the above-mentioned specific fine powder or alumina is not effective in improving the scratch resistance of polyester film.